Sergei Skripal would not have survived if a military-grade agent had been used, says Russian President Vladimir Putin.

Former Russian spy Sergei Skripal, whose poisoning with a nerve agent in the UK triggered a diplomatic crisis with Russia, has been discharged from a British hospital.

The 66-year-old, a former colonel in Russia's military intelligence, and his daughter Yulia were found unconscious on a bench in the southern English city of Salisbury on March 4.

Accusations by Britain that Moscow was behind the nerve agent attack led to a Russia-West crisis in which Western governments, including the United States, expelled more than 100 Russian diplomats.

Russia has denied any involvement in the poisoning and retaliated with the expulsion of Western diplomats.

The father and daughter were in a critical condition for weeks and doctors at one point feared that, even if they survived, they might have suffered brain damage.

But their health began to improve rapidly, with Yulia discharged last month and her father on Friday.

"It is fantastic news that Sergei Skripal is well enough to leave Salisbury District Hospital," the hospital's Chief Executive Cara Charles-Barks said in a statement.

"Treating people who are so acutely unwell, having been poisoned by nerve agents, requires stabilising them, keeping them alive until their bodies could produce more enzymes to replace those that had been poisoned," the hospital statement read.

Souring relations

UN experts confirmed in April that a high-purity nerve agent was used but did not specify whether it was Novichok, a nerve agent allegedly developed in the Soviet Union, or who might be behind the poisoning.

Russian President Vladimir Putin said he was "very happy" about Skripal's hospital discharge, but used the occassion to denounce accusations that Russia had poisoned the ex-spy with a military-grade nerve agent.

"I think if a military-grade poisonous agent had been used, as our British colleagues stated, this person would have died instantly," Putin said during a press conference with German Chancellor Angela Merkel.

Russia has challenged Britain to provide evidence in the case and has requested consular access to Skripal and his daughter.

Following Skripal's release, Britain's Metropolitan Police said it was making "good progress" in the case.

"This is a complex investigation and detectives continue to gather and piece together all the evidence to establish the full facts and circumstances behind this dreadful attack," it said in a statement.

Protective custody

The Russian embassy in London has accused Britain of blocking access to the pair in defiance of diplomatic convention.

Alexander Yakovenko, Russia's ambassador to London, welcomed Skripal's progress on Friday but reiterated Russia's desire to see the pair, saying that currently Britain was not fulfilling its obligations under international law.

"We're happy that he's alright," Yakovenko told reporters.

"We're still demanding the access to these people. We want just to understand how they feel and we want them to say personally what they want. If they don't want our assistance it's fine. We want to see them physically."

The Foreign and Commonwealth Office said their offer of consular assistance had been passed to Yulia Skripal, a Russian citizen, but that she had turned it down.

Both she and her father are now likely to be in protective custody.

"In the interests of Sergei and Yulia's safety, we will not be discussing any protective or security arrangements that are in place," the Metropolitan Police said.